The Meter ID is accessible on the "Manage Bills" page.
Go to the Energy (or Water/Waste tab), and click on the meter in question.This will take you to the "Manage Bills" page, then expand the caret next to "Basic Meter Information."
The Meter ID is also available in property and portfolio download file.
The Meter ID is specific to Portfolio Manager and cannot be changed. However, you can change the meter name.